Research Uses New Stream Monitoring Method to Identify Elevated Groundwater Methane in Shale-Gas Development Area
Posted: May 3, 2015
The potential for ground water and surface water impacts from upward migration of fluids injected into shales during hydraulic fracturing appears to be negligible based on the best available science and data. However a mechanism for ground water contamination during drilling is possible if stray gases are mobilized during the drilling process along preferential pathways, such as boreholes, faults and fractures. Methane generated from black shales (known as thermogenic methane) can migrate from shallow gas-bearing formations into overlying aquifers as dissolved gas naturally over very long periods of time or more rapidly during the drilling and well construction process. Methane-laden ground water from these aquifers may eventually discharge to wells, springs, or gaining streams and be detected if the gas occurs at sufficiently high concentrations. A team of researchers from Penn State University , University of Utah, and the US Geological Survey utilized novel methods to collect samples and measure the concentration of methane in fifteen streams in northeastern Pennsylvania. The method has the potential to not only locate natural methane discharges, but also quantify increased fluxes related to shale-gas development. This stream-based monitoring approach integrates ground water information from gaining stream reaches over km-scale distances due to regional convergence of ground water flow. A new concept is tested in this paper by combining stream hydrocarbon and noble-gas measurements with reach mass-balance modeling to estimate thermogenic methane concentrations and fluxes in groundwater discharging to streams and to constrain methane sources. The research team sampled methane in 15 streams as a reconnaissance tool to locate methane-laden groundwater discharge: concentrations up to 69 parts per billion (ppb) were observed, with four streams ≥5 ppb. Geochemical analyses of water from one stream with high methane (Sugar Run, Lycoming County) were consistent with Middle Devonian gases. After sampling was completed, it was learned that the state regulator was investigating possible stray-gas migration from a nearby Marcellus Formation gas well. Modeling indicates a ground water thermogenic methane flux of about 0.5 kg/d discharging into Sugar Run, possibly from this fugitive gas source. The methane level of 69 ppb detected in Sugar Run is a relatively low concentration, nearly 100 times less than the state regulator’s action level of 7,000 ppb. This method appears promising to determine where and how hydrocarbons may enter freshwater resources and how often such migration is caused by shale-gas development.
The article was recently published in Environmental Science and Technology